The invention relates to a safety method and apparatus for a perforating gun.
Referring to FIG. 1, a typical perforating gun string 10 may have several perforating guns 12. Each perforating gun 12 may have phased shaped charges 14 that are used to penetrate a casing of a subterranean well and form fractures in surrounding formations to enhance the production of well fluids from these formations. Because the shaped charges 14 may potentially inflict harm if the charges 14 prematurely detonate, several safety mechanisms typically are used to prevent accidental detonation of the shaped charges 14.
For example, the shaped charges 14 may be secondary explosives that, as compared to primary explosives, are very difficult to detonate. To detonate these secondary explosives, the perforating gun string 10 may have a firing head 11 that is associated with each perforating gun 12. In this manner, the firing head 11 may include a detonator 15 that, when activated, detonates a primary explosive to initiate a shockwave on a detonating cord 17 that extends to the shaped charges 14. The shockwave, in turn, propagates down the detonating cord 17 and detonates the shaped charges 14. Because the primary explosive is capable of being detonated by naturally occurring phenomena, extreme care must be exercised to ensure that the primary explosive does not prematurely detonate.
The detonation of the perforating gun 12 may be remotely controlled from the surface of the well. To accomplish this, stimuli may be transmitted downhole to the firing head 11 to cause the detonator 15 to initiate the shockwave on the detonating cord 17. As examples of techniques that are used to transmit the stimuli, an internal passageway of the string 10, an annulus that surrounds the string 10, a tubing of the string 10, or a line (a slickline or a wireline, as examples) extending downhole may all be used. Other techniques may also be used to transmit command stimuli downhole.
Detonation of the primary explosive typically requires energy from an energy source, a source that may either be located at the surface of the well or downhole in the perforating gun string 10. If the energy source is at the surface of the well, then an operator may disconnect the energy source until firing of the perforating guns 12 is desired. However, unfortunately for the other case, connection/disconnection of a downhole energy source may present difficulties, as circuitry (not shown) of the firing head 11 must connect/disconnect the energy source. For example, a battery 16 of the string 10 may provide the energy needed to cause the detonator 15 to initiate a shockwave on the detonating cord 17. However, a problem with this arrangement is that the battery 16 is located downhole with the detonator 15. Thus, if the circuitry that couples the battery 16 to the detonator 15 should fail, the shaped charges 14 may be inadvertently detonated.
Thus, there is a continuing need for a downhole safety system to reliably prevent accidental detonation of a perforating gun when an energy source to detonate the gun is located downhole with the gun.